IE8: RadioButton tab order wrong

Description

Load test_CheckBox.html in IE8. TAB thru the radio button group containing news/talk/weather. The widget "news" is focused but "talk" should have been since it's checked. This is affecting a11y compliance. Seems to be related to IE not properly processing the checked attribute until the node is attached to body (after startup()) but the attributes are applied before startup().

Refs #16621, #16512. Blank RadioButton? value was being set to "on" after reset which is incorrect (but correct for CheckBox? widgets). Removed unnecessary set/removeAttribute(checked) calls since only domAttr.set is needed for the checked attribute. Don't call domAttr.set if already set correctly to keep IE from losing tab order. Added automated tests.